There is considerable interest in the rapid assessment of average body density, particularly for athletes, insofar as average body density provides an indication of total body fat. For example, percent body fat may be estimated according to the volume formula: EQU % fat=(4.57/density-4.14)*100.
Such density measurements are currently made by the so-called hydrostatic method in which the individual is submerged in water and the volume of water displaced by the individual's body is determined providing an indication of the individual's total body volume. Total submersion of an individual, as is required by this method, is inconvenient and stressful to the individual.
Another drawback to the hydrostatic method is the fact that the volume so measured includes the air-filled lungs and respiratory track. Including these air-filled volumes erroneously lowers the apparent density of the individual's tissues. Whereas fat is approximately nine-tenths as dense as nonfat tissue, air is approximately 1000 times less dense than nonfat tissue. Thus, even small amounts of measured air can provide serious error in the calculation of density when used to compute percentage of body fat.
Accordingly, interest has developed in less severe methods of measuring body volume. One method encloses the individual in an airtight chamber connected to a second pressurized chamber of known volume. Pressure measurements are made in the chamber housing the individual both before and after a valve is opened connecting that chamber to the second pressurized chamber. Knowing two pressures and the incremental volume of the second chamber permits one to calculate the volume of the individual contained in the first chamber. This calculation employs Boyle's law which states that for a given amount of gas at a constant temperature, the pressure of the gas times its volume will be constant.
The second chamber in this method provides an accurate way of increasing the total volume of the system, but is cumbersome and slow. The second chamber must be pre-pressurized and measurements are only made after the first and second chamber have reached a pressure equilibrium. For use with a human, and for significant pressure changes, this speed of equalization must be moderated to prevent discomfort to the individual, particularly with respect to pressure across the individual's eardrum.
These problems of low measurement speed are avoided by methods that subject the patient to subsonic pressure fluctuations. By using a dynamic measurement process, small leaks in the chamber may be tolerated and thus the need for a separate pre-pressurized volume can be avoided. Nevertheless, these dynamic techniques are generally restricted to small amplitude pressure variations and thus have less potential for high accuracy measurement. Further, the devices are complex in construction.